Sepsis affects nearly 20 million people per year with a mortality rate of 30-40%[1]. These patients require intensive care with associated high costs, which impose significant health-care, economic, and social burdens. In particular, each septic patient in the United States incurs costs of approximately $25,000 during hospitalization, totaling a nationwide annual bill in excess of $17 bn[1]. The time to initiation of effective antimicrobial therapy is known to be the single strongest predictor of outcome, as every hour delay in its administration increases by 8% the risk of death[2]. Besides the obvious economic benefits, the development of fast, accurate, and inexpensive diagnostic methods thus appears as a major goal for alleviating human pain. Microbial culture remains the most widespread technique for identifying the infectious agent, but unfortunately it requires 24-72 hours to provide a conclusive diagnosis for common infections. Understandably, a large deal of work has been devoted over the last three decades to developing alternative methods for fast identification of bacteria in suspected patients[3]. These methods include immunology-based approaches (e.g., enzyme-linked immunosorbent assay (ELISA), as well as fluorescence and radio immunoassays)[4], nucleic acid identification (e.g., polymerase chain reaction (PCR))[5], and spectrometry-based procedures (e.g., matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry)[6]. Although generally faster than just sequential microbial culture, these techniques still require hours to days, depending on the pathogen. Additionally, immunological and nucleic acid tests are expensive (around $200 each) and monoplex (one test per target microorganism), while MALDI-TOF still relies on microbial culture to isolate pure colonies. As a consequence, a broad-spectrum antibiotic is generally recommended to cover all potential pathogens until obtaining a conclusive identification. Apart from its inherent cost and adverse health effects, this indiscriminate use of antibiotics induces bacterial resistance, a growing problem of modern pharmacopeia[7]. Society therefore urges for the development of new diagnostic systems capable of providing fast, accurate, inexpensive, and if possible multiplexed identification of infectious agents in body fluids[8].
The same problems arise with any kind of analyte instead of microorganisms, since rapid and easy identification and quantification of analytes on fluids, for instance a sample of blood, is of the outmost importance.
Recent advances in nanoscience, spectroscopy, magnetism, plasmonics, and microfluidics have generated great expectations for the development of new approaches to bacteria detection[9-12]. Unfortunately, the methods so far proposed are generally time consuming, capable of only exploring small sample volumes (˜microliters), working exclusively for one a priori selected pathogen[9], not truly multiplex[10], requiring from external labels[11], or relying on additional steps to record a suitable signal for identification[12].
Therefore, there is still a need for improved methods and systems for identification and quantification of analytes in fluids reducing the complexity, length of time and allowing working with a relatively large volume sample.